Can anyone help me figure out what a hissing sound from the bottom of the car could be? I haven't been able to pinpoint the exact location of the sound but it looks to be isolated in the rear of the car. Can't quite seem to locate it though. Does anyone know what this sound might be? I first heard it after turning my car on (after 2-3 days of sitting) and I was walking around the car to check my tire pressures while the car warmed up alittle and I heard a funny hissing sound from the bottom of the car. If you can help me diagnose what this is, I'd really appreciate it! Thanks in advance for your help.

most likely an exhaust leak.

Get yourself a long soft tube-like material and put it up to area you think the leak is emanating from and listen for it through the other end of the tube.

most likely a bad flex pipe....

is horsepower lost from bees in a can???(SOrry to steal the thread...)

only if there is a hole along the exhaust pipe your accel would be weak

ummm could be fuel pump? just happened to start noticing it now or something

I just started noticing it now. I have two trouble codes that I have yet to fix. One of them is the 0510 (rear o2 sensor) and the other is the 0302 (excessive or insufficient EGR flow). I doubt that any of these trouble codes have anything to do with the hissing but I could wrong. Any thoughts?

a friend of mine just painted his max and its shinier than half these, and he hasnt even waxed it or anything

wtf happened i hate this laptop mouse

i hear the same thing under my MAX i thought it was the AC, if it is a hole in the exaust pipe how could I fix/patch it?

-Dexter

Do you have an aftermarket exhaust? Your rear oxygen sensor could be loose. Try screwing it back into the hole.

if it were the fuel pump wouldn't you be able to smell gas leaking?

-Dexter

I have a full stock 99 SEL except for K&N panel filter. I'm thinking maybe the o2 sensor could be loose.